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For nuclear magnetic resonance determinations of the conformation of oligosaccharides in solution, simple molecular mechanics calculations and nuclear Overhauser enhancement measurements are adequate for small oligosaccharides that adopt single, relatively rigid conformations. Polysaccharides and larger or more flexible oligosaccharides generally require additional types of data, such as scalar and dipolar coupling constants, which are most conveniently measured in 13C-enriched samples. Nuclear magnetic resonance relaxation data provide information on the dynamics of oligosaccharides, which involves several different types of internal motion. Oligosaccharides complexed with lectins and antibodies have been successfully studied both by X-ray crystallography and by nuclear magnetic resonance spectroscopy. The complexes have been shown to be stabilized by a combination of polar hydrogen bonding interactions and van der Waals attractions. Although theoretical calculations of the conformation and stability of free oligosaccharides and of complexes with proteins can be carried out by molecular mechanics methods, the role of solvent water for these highly polar molecules continues to present computational problems.

To probe differences in conformation of the type 1 and type 2 linkages in blood group oligosaccharides, two-dimensional nuclear Overhauser effect spectroscopy (2D-NOESY) and 1H T1 data were obtained for two blood group A oligosaccharide alditols containing the type 1 and type 2 linkage. The NOE data were interpreted using a complete relaxation matrix approach. Simulations of NOE and T1 values were made using disaccharide and tetrasaccharide model conformations generated by a systemic variation of the glycosidic dihedral angles phi and psi. NOEs from the amide protons of GlcNAc and GalNAc in the type 1 pentasaccharide alditol were obtained, and simulated in a manner similar to those from carbon-bound protons. In addition to providing data for determining the conformation of the type 1 linkage from amide proton NOEs of GlcNAc and GalNAc to neighboring residues, amide proton NOEs also yield information on the orientation of the acetamido side chains. The amide NOE data indicated subtle differences in the orientation of the amide side chain of GlcNAc among the A type 1 pentasaccharide alditol and two previously studied blood group oligosaccharides, lacto-N-difucohexaose 1 and lacto-N-fucopentaose 1. From the NOE and 1H T1 data, and from simple rigid geometry energy calculations, it is concluded that the type 1 and type 2 linkages in the oligosaccharides studied have different conformations and that these conformations are relatively rigid in solution.

In order to search for probable conformations of the peptide, the amino acid side chain, and the carbohydrate linkage in glycoproteins, conformational energy surfaces of glycopeptide model compounds were studied by Monte Carlo methods using the Metropolis algorithm. The potential energies were composed of empirical energy functions which include nonbonded interactions, electrostatics, hydrogen bonding, and torsional energies specified by parameters which have been used for peptides. Calculations were performed on 1-N-acetyl-2-acetamido-beta-D-glucopyranosyl amine and the glycosylated dipeptide N-acetyl-delta-N-(2-acetamido-beta-D-glucopyranosyl)-L-asparaginyl-N'-methyl amide as models for N-glycosylated peptides and on methyl-2-acetamido-alpha-D-galactopyranoside as well as the glycosylated dipeptides N-acetyl-gamma-O-(2-acetamido-alpha-D-galactopyranosyl)-L-threonyl-N'-methyl amide and its seryl analog as models for O-glycosylated glycoproteins. The probable conformations of these compounds were analyzed by single-angle probability tables and by two-dimensional conformation density maps projected from the Markov chains which contained up to six independently varied conformational dihedral angles. The presence of high barriers to rotation required the use of search strategies which resulted in a rather low acceptance rate for new conformations in the Metropolis algorithm in order to avoid trapping of the Markov chain in local energy minima. This problem contributed to the failure of these calculations to attain complete convergence to the thermodynamic limit for the glycosylated dipeptide models in which six dihedral angles were independently varied. Analysis of the results shows that the conformational space available to the highly crowded axial glycosides of the alpha-O-GalNAc type is much more restricted than that for the N-asparaginyl glycopeptides. The most probable conformation for the O-glycosylated peptides is is a beta-turn while N-glycosylated peptides may be either in a beta-turn or an extended conformation.

Treatment of a blood group A-active ovarian cyst mucin glycoprotein with alkaline borohydride under conditions expected to cleave O-glycosidic linkages between carbohydrate and peptide releases a sulfated polysaccharide of average molecular weight 20,000. Its peptide and mannose content is less than 1%, and carbohydrate analysis gives Fuc/GalNAc/Gal/GlcNAc in the ratio of 1:1:2.2:2.2. Galactosaminitol is recovered at the level of one residue per 112-residue average polysaccharide chain. The 13C- and 1H-NMR spectra show that the polysaccharide has side chains whose non-reducing terminals have the blood group A structure on a type 1 chain: (Formula: see text). Methylation analysis confirms the presence of these blood group A type 1 sidechains as well as 4-substituted GlcNAc, 3-substituted galactose and 3,6-substituted galactose branch points. Periodate oxidation removes all the fucose and GalNAc from the non-reducing terminal but leaves intact the backbone composed of beta-linked Gal and GlcNAc, as would be expected for a polylactosamine. Although the native polysaccharide is resistant to endo-beta-galactosidase digestion, the product of periodate degradation is partially digested, giving a 30% yield of a trisaccharide shown by 1H-NMR spectroscopy to be: Gal(beta 1----3)GlcNAc(beta 1----3)Gal We conclude that this is a high molecular weight sulfated polysaccharide which is related to the asparagine-linked polylactosamine chains of cell surface glycoproteins which have been implicated in cell differentiation. However, the blood group A polysaccharide from the ovarian cyst mucin is unique in several respects. It is linked to the protein by an O-glycosidic bond rather than the N-asparagine linkage of the previously known polylactosamines which have a trimannosyl core, and its blood group A side chains are on a type 1 core rather than type 2 which is found on other polylactosamines.

Circular dichroism spectra (CD) of an active fraction of antifreeze glycoprotein (AFGP) and of a lower molecular weight fraction which is less active are found to be similar in the wavelength range 170-230 nm. The contribution of the disaccharide side chain chromophores to the CD spectra of the glycoprotein is estimated from spectra of model oligosaccharides having similar structures. Comparison of the CD spectra of the peptide portion of AFGP to those of established model polypeptides suggests a 3-fold left-handed helix of the collagen type as the most likely conformation of the peptide. In the proposed model, the disaccharide moieties attached to every third residue are in identical environments. Furthermore, proline residues which are found in the less active fraction can be accommodated in this helical structure, consistent with the observed similarity of the spectra of the two fractions.

Circular dichroism spectroscopy in the 170-220-nm range of the ultraviolet and measurements of the amide proton coupling constants in nmr have been used to investigate the conformation of asparagine-linked glycopeptides having oligosaccharide chains of the complex antenna type. The CD spectra can be explained as the sum of three contributions, the first of which is a pair of large bands of opposite sign resulting from coupling of the adjacent chromophores of the 2-acetamido-1-N-(4-aspartyl)-2-deoxy-beta-D-glucopyranosylamine linkage. Secondly the amide chromophore of the core N-acetylglucosamine residue substituted at carbon 4 by a beta-mannosyl residue contributes a negative band at 210 nm along with a small negative signal in the 180-190-nm region. The amides of the antenna N-acetylglucosamine residues, which in some cases are substituted by beta-galactosyl residues at carbon 4, contribute a negative band at 210 and a strong positive band at 185-190 nm. Since these three contributions are approximately independent, we can detect no long range interactions between nonreducing termini and the glycopeptide linkage region. Both CD and nmr data imply that the glucosaminyl-asparagine linkage is rigidly fixed in a conformation having the amide protons trans to the sugar ring protons. These results are consistent with an extended conformation in the shape of a "Y" or a "T" for complex type asialo-oligosaccharide chains.

An Arg-containing antifreeze glycoprotein from the polar fish Eleginus gracilis was isolated, and the major components were purified to homogeneity. The general protocol for purification was chromatography of serum on DEAE-cellulose, followed by chromatography on a cation exchanger. DEAE-cellulose chromatography resulted in two fractions, A and B. Fraction A contained most of the antifreeze glycoprotein found in E. gracilis (approximately 80% by weight) and consisted of 13 distinct components. Unlike antifreeze glycoproteins from other previously studied polar fish, Fraction A contained both low and high molecular weight antifreeze glycoprotein components. The two major components of Fraction A were sequenced and compared with the sequence of antifreeze glycoproteins 7 and 8 from both Boreogadus saida and Pagothenia borchgrevinki. The antifreeze glycoproteins from E. gracilis were shown to have a similar composition to those previously studied, except for an additional Ala-Arg dipeptide at the carbon terminal in the major components of Fraction A and the position of Pro in the low molecular weight components. The activity of E. gracilis antifreeze glycoproteins is the subject of a companion article (Burcham, T. S., Osuga, D. T., Yeh, Y., and Feeney, R. E. (1986) J. Biol. Chem. 261, 6390-6397).

High-field proton and 13C NMR spectroscopy has been used to test and refine the recent proposal, based on vacuum uv circular dichroism results, of a threefold left-handed helical conformation for antifreeze glycoprotein (AFGP). Partial assignment of the protons of the glycotripeptide repeating unit has been made by comparison with spectra of model compounds, by selective decoupling, and by measurements of nuclear Overhauser effect (nOe). At 40 degrees C, AFGP fraction 8 (Mr 2600) shows 2-Hz linewidths which broaden at lower temperature. Neither 1H nor 13C chemical shifts depend strongly on temperature, suggesting no abrupt conformational transition. The nOe between alanine alpha and beta protons vary with temperature and with field strength, from small positive enhancements at 50 degrees C and 80 MHz to large negative effects at 3 degrees C and 300 MHz, indicating a substantial change of rotational correlation time with temperature. The higher-molecular-weight fraction 1-4 shows negative nOe at all temperatures. The CD spectra of fraction 1-4 show bands characteristic of the polyproline II structure at both 3 and 50 degrees C, while those bands in fraction 8 are weaker at 50 than 3 degrees C. The 1H nOe, the 13C T1, and CD data are interpreted as indicating that AFGP fraction 8 is an extended "rod-like" conformation at low temperature which becomes a flexible coil at high temperature, while fraction 1-4 is a flexible rod with sufficient segmental mobility to eliminate any long-range order.

With its simple glycotripeptide repeating structure the antifreeze glycoprotein of polar fish may be an especially simple conformational mode for mucin glycoproteins with similar but more complex structures. The fully assigned proton n.m.r. spectrum confirms the anomeric configurations of the hexapyranosidic sugars of the side chains and the coupling constants of the alpha GalNAc and the beta Gal residues show both to be in the expected 4C1 chair conformation. The assignment of a single resonance for each proton of the (Ala-Thr-Ala)n repeat unit coupled with the observation of long range nuclear Overhauser effects (n.O.e.) implies a three-fold repeating conformation. The resonances of the two alanines are distinct and can be assigned to their correct positions in the peptide sequence by n.O.e. observed at the amide proton resonances on saturation of the alpha proton signals. The amide proton coupling constants of all three peptide residues are similar and imply a limited range of peptide backbone torsion angles, phi CN. The large n.O.e. which has been observed between the amide proton and the alpha proton of the residue preceding it in the sequence implies large positive values for the peptide dihedral angle, psi CC. Limits are placed on possible values of side chain dihedral angles by the observation of the coupling constant between the alpha and beta protons of the threonyl residue. The observation of n.O.e. between the anomeric proton of GalNAc and the threonyl side chain protons gives information on the conformation of the alpha glycosidic linkage between the disaccharide and the peptide. n.O.e. observed between the protons of the beta glycosidic linkage indicates the conformation of the disaccharide and the large amide proton coupling constant of the GalNAc residue shows a trans proton relationship. The spectroscopically derived data have been combined with conformational energy calculations to give a conformational model for antifreeze glycoprotein in which the hydrophobic surfaces of the disaccharide side chains are wrapped closely against a three-fold left handed helical peptide backbone. The hydrophilic sides of the disaccharides are aligned so that they may bind to the ice crystal face, which is perpendicular to the fast growth axis inhibiting normal crystal growth.